This invention relates to apparatus for aligning vehicle wheels, and more particularly to radio frequency communication systems for such apparatus.
Apparatus for aligning vehicle wheels typically use sensors mounted at each wheel and a central console (or a handheld remote unit) which receives and manipulates data from the sensors. Communication between the sensors and the console (or remote unit) is conventionally accomplished over electrical cables. Cables, however, do have certain drawbacks. They tend to get in the way, they must be physically installed by the user, and their length is fixed (requiring consistent placement of the vehicle). On the other hand, cables provide a reliable communications channel and are relatively inexpensive.
At least one cordless communications system has been proposed. This system, described in U.S. Pat. No. 4,761,749 to Titsworth et al., uses infrared light beams for communications (although the reference states without explanation that radio frequencies could also be used). Infrared systems have proved to be difficult to implement in practice. They suffer from interference from sunlight and they typically require line-of-sight orientations to function properly. Moreover, the baud rate (transfer rate) of economical IR systems is slower than desirable.
Radio frequency (RF) systems also suffer from potential drawbacks. Such systems can be much more expensive than cable-based systems. Radio waves, particularly at the higher frequencies, undergo multiple reflections, and their transmission paths can even be affected by a technician/user walking about the service area. Moreover, there are numerous possible sources of signal error, particularly if the desired turnaround time between sensor and console (or remote unit) is rather short. Particularly at higher frequencies, shielding between units is difficult, expensive, or both. Data misinterpretation is also a significant drawback with RF systems. For example, signals from one sensor unit may be misinterpreted by another sensor unit as a signal from the console (or remote unit). Similarly, noise may be misinterpreted as signal.